Semiconductor device and method for fabricating the same

ABSTRACT

The invention primarily provides gate electrodes and gate wirings permitting large-sized screens for active matrix-type display devices, wherein, in order to achieve this object, the construction of the invention is a semiconductor device having, on the same substrate, a pixel TFT provided in a display region and a driver circuit TFT provided around the display region, wherein the gate electrodes of the pixel TFT and the driver circuit TFT are formed from a first conductive layer, the gate electrodes are in electrical contact through connectors with gate wirings formed from a second conductive layer, and the connectors are provided outside the channel-forming regions of the pixel TFT and the driver circuit TFT.

This application is a divisional of copending U.S. application Ser. No.11/396,436 filed on Apr. 3, 2006 which is a divisional of U.S.application Ser. No. 09/544,801 filed on Apr. 7, 2000 now U.S. Pat. No.7,456,430.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device provided with anactive circuit comprising a thin-film transistor (hereunder referred toas “TFT”) on a substrate with an insulating surface. The invention maybe used with particular advantages in electro-optical devices, a typicalone being a liquid crystal display device having an image display regionand its driver circuit formed on the same substrate, and inelectro-optical device-mounted electronic instruments. Throughout thepresent specification, “semiconductor device” will refer to generaldevices that function based on semiconductor properties, and it willinclude in its scope the aforementioned electro-optical devices andelectronic devices having the electro-optical devices mounted thereon.

2. Description of the Related Art

TFTs having semiconductor layers formed with crystalline silicon films(hereunder referred to as “crystalline silicon TFTs”) have highfield-effect mobility, and are therefore capable of forming circuitswith various functions. Active matrix-type liquid crystal displaydevices employing crystalline silicon TFTs have an image display regionand a driver circuit for image display formed on the same substrate. Inthe image display region there are provided a pixel TFT formed by ann-channel TFT, and a storage capacitor, while the driver circuit isconstructed with a shift register circuit, level shifter circuit, buffercircuit, sampling circuit or the like, which is formed based on a CMOScircuit.

However, the operating conditions are not the same for the pixel TFT andthe driver circuit TFT, and therefore different properties are oftenrequired for the TFTs. For example, the pixel TFT functions as a switchelement and is driven by application of a voltage to the liquidcrystals. Because the liquid crystals are driven by alternating current,it is most common to employ what are known as frame inversion drivingsystems. In such systems, the pixel TFT is required to have the propertyof a sufficiently low off-state current value (the drain current flowingwhen the TFT is off) in order to minimize power consumption. On theother hand, since a high driving voltage is applied to the buffercircuit of the driver circuit, it is necessary to increase the voltageresistance to prevent breakage upon application of the high voltage.Increased current driving capacity requires a sufficient guarantee forthe on-state current value (the drain current flowing when the TFT ison).

The lightly doped drain (LDD) structure is known as a structure for aTFT exhibiting a reduced off-state current value. This structure isprovided with a region having an impurity element added at a lowconcentration between a channel-forming region and a source region ordrain region formed by addition of an impurity element to a highconcentration, and this region is called the “LDD region”. One meansknown for preventing deterioration of the on-state current value due tohot carriers is a structure known as a GOLD (Gate-drain Overlapped LDD),wherein the LDD region is placed lying over the gate electrode with agate insulating film therebetween. This type of structure is known to beeffective for preventing inclusion of hot carriers by attenuation ofhigh voltage near the drain, thus avoiding the deterioration phenomenon.

At the same time, demands are increasing for larger sized and moreintricate screens, to give greater product value to active matrix-typeliquid crystal display devices. However, the larger sizes and greaterintricacy of screens increases the number and length of the scanninglines (gate wirings), thus heightening the necessity for low resistanceof the gate wirings. That is, as the number of scanning lines increases,the charging time for the crystals is shortened, such that the timeconstant for the gate wiring (resistance×capacity) must be reduced for afaster response. For example, if the resistivity of the material formingthe gate wiring is 100 μΩcm the limit to the screen size will be about 6inches, but for 3 μΩcm a display corresponding to 27 inches is possible.

Still, the properties required for a pixel TFT of a pixel matrix circuitand a TFT of a driver circuit such as a shift register circuit or buffercircuit are not always the same. For example, in a pixel TFT, a largereverse bias (a negative voltage in the case of an n-channel TFT) isapplied to the gate, but a driver circuit TFT will basically fail tooperate in a reverse bias state. The operating speed of a pixel TFT isalso sufficient at less than 1/100 that of a driver circuit TFT.

In addition, while a GOLD structure provides a strong effect ofpreventing on-state current value deterioration, it has also presentedthe problem of a larger off-state current value compared to the usualLDD structure. Thus, it has not been a preferred structure forapplication to pixel TFTs. Conversely, the usual LDD structure has astrong effect of minimizing the off-state current value but has had alow effect of preventing deterioration due to hot carrier inclusion byattenuation of the electric field near the drain. Consequently, it hasnot always been preferable to form all the TFT with the same structurein semiconductor devices comprising multiple integrated circuits withdifferent operating conditions, such as active matrix-type liquidcrystal display devices. These problems have become more conspicuousparticularly in crystalline silicon TFTs with higher characteristics,and as greater performance has been required for active matrix-typeliquid crystal display devices.

The use of aluminum (Al) and copper (Cu) as wiring materials has beenconsidered for realization of large-sized active matrix-type liquidcrystal display devices, but this has presented drawbacks such as poorcorrosion resistance and heat resistance. Consequently, these materialsare not necessarily preferred for formation of TFT gate electrodes, andit has not been easy to introduce such materials into the TFTmanufacturing process. Wirings can of course be formed with otherconductive materials, but there are no materials with such lowresistance as aluminum (Al) and copper (Cu), and this has hamperedfabrication of large-sized display devices.

SUMMARY OF THE INVENTION

In order to solve the problems discussed above, the construction of thepresent invention is that of a semiconductor device having, on the samesubstrate, a pixel TFT provided in a display region and a driver circuitTFT provided around the display region, wherein the pixel TFT and thedriver circuit TFT have gate electrodes formed from a first conductivelayer, the gate electrodes are in electrical contact through connectorswith gate wirings formed from a second conductive layer, and theconnectors are provided outside the channel-forming regions of the pixelTFT and the driver circuit TFT.

Another construction of the invention is that of a semiconductor devicehaving, on the same substrate, a pixel TFT provided in a display regionand a driver circuit TFT provided around the display region, wherein thepixel TFT and the driver circuit TFT have gate electrodes formed from afirst conductive layer, the gate electrodes are in electrical contactwith gate wirings formed from a second conductive layer, throughconnectors provided outside the channel-forming regions of the pixel TFTand the driver circuit TFT, the LDD regions of the pixel TFT aredisposed without overlapping the gate electrode of the pixel TFT, theLDD regions of the first n-channel TFT of the driver circuit aredisposed so as to overlap the gate electrode of the first n-channel TFT,and the LDD regions of the second n-channel TFT of the driver circuitare disposed so that at least a portion thereof overlaps the gateelectrode of the first n-channel TFT.

In this construction of the invention, the first conductive layer has aconductive layer (A) containing nitrogen and at least one selected fromamong tantalum, tungsten, titanium and molybdenum, a conductive layer(B) formed on the conductive layer (A) and composed mainly of at leastone selected from among tantalum, tungsten, titanium and molybdenum anda conductive layer (C) formed on the areas where the conductive layer(B) does not contact the conductive layer (A) and containing nitrogenand at least one selected from among tantalum, tungsten, titanium andmolybdenum, while the second conductive layer has a conductive layer (D)composed mainly of aluminum or copper and a conductive layer (E)composed mainly of at least one selected from among tantalum, tungsten,titanium and molybdenum, and the conductive layer (C) and conductivelayer (D) are in contact at the connectors. The conductive layer (B)contains argon as an added element, and the oxygen concentration in theconductive layer (B) is 30 ppm or less.

In order to solve the aforementioned problems, the method forfabricating a semiconductor device according to the invention is amethod for fabrication of a semiconductor device having, on the samesubstrate, a pixel TFT provided in a display region and a driver circuitTFT provided around the display region, the method comprising a step offorming gate electrodes for the pixel TFT and the driver circuit TFTfrom a first conductive layer, and a step of forming gate wiringsconnected to the gate electrodes from a second conductive layer, whereinthe gate electrodes and the gate wirings are connected throughconnectors provided outside the channel-forming regions of the pixel TFTand the driver circuit TFT.

The method for fabricating a semiconductor device according to theinvention is also a method for fabrication of a semiconductor devicehaving, on the same substrate, a pixel TFT provided in a display regionand a driver circuit TFT provided around the display region, the methodcomprising a first step of selectively adding an n-type impurity elementto the first and second n-channel TFT semiconductor layers forming thedriver circuit to a concentration range of 2×10¹⁶ to 5×10¹⁹ atoms/cm³, asecond step of forming gate electrodes for the pixel TFT and the drivercircuit TFT from a first conductive layer, a third step of selectivelyadding a p-type impurity element to the p-channel TFT semiconductorlayers forming the driver circuit to a concentration range of 3×10²⁰ to3×10²¹ atoms/cm³, a fourth step of selectively adding an n-type impurityelement to the first and second n-channel TFT semiconductor layersforming the driver circuit and the semiconductor layer of the pixel TFTto a concentration range of 1×10²⁰ to 1×10²¹ atoms/cm³, a fifth step ofselectively adding an n-type impurity element to the semiconductor layerof the pixel TFT to a concentration range of 1×10¹⁶ to 5×10¹⁸ atoms/cm³,using at least the gate electrode of said n-channel TFT as a mask, and asixth step of forming gate wirings for the pixel TFT and the drivercircuit TFT from a second conductive layer, wherein the gate electrodesand the gate wirings are connected through connectors provided outsidethe channel-forming regions of the pixel TFT and the driver circuit TFT.

In the method for fabrication of a semiconductor device according to theinvention,

the first conductive layer is formed by a step of forming a conductivelayer (A) containing nitrogen and at least one selected from amongtantalum, tungsten, titanium and molybdenum, a step of forming aconductive layer (B) formed on the conductive layer (A) and composedmainly of at least one selected from among tantalum, tungsten, titaniumand molybdenum, and a step of forming a conductive layer (C) formed onthe areas where the conductive layer (B) does not contact the conductivelayer (A) and containing nitrogen and at least one selected from amongtantalum, tungsten, titanium and molybdenum, while the second conductivelayer is formed by at least a step of forming a conductive layer (D)composed mainly of aluminum or copper and a step of forming a conductivelayer (E) composed mainly of at least one selected from among tantalum,tungsten, titanium and molybdenum, and conductive layer (C) andconductive layer (D) are in contact at the connectors. The conductivelayer (A) may be formed by a sputtering method using a target composedmainly of at least one selected from among tantalum, tungsten, titaniumand molybdenum, in a mixed atmosphere of argon and nitrogen or ammonia,and the conductive layer (C) is preferably formed by heat treatingconductive layer (B) in a nitrogen atmosphere with an oxygenconcentration of 1 ppm or less. Conductive layer (C) may also be formedby heat treating conductive layer (B) in a nitrogen plasma atmospherewith an oxygen concentration of 1 ppm or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are cross-sectional views of fabrication steps for apixel TFT, a storage capacitor and a driver circuit TFT;

FIGS. 2A to 2D are cross-sectional views of fabrication steps for apixel TFT, a storage capacitor and a driver circuit TFT;

FIGS. 3A to 3D are cross-sectional views of fabrication steps for apixel TFT, a storage capacitor and a driver circuit TFT;

FIGS. 4A to 4C is cross-sectional views of fabrication steps for a pixelTFT, a storage capacitor and a driver circuit TFT;

FIG. 5 is a cross-sectional view of a pixel TFT, a storage capacitor anda driver circuit TFT;

FIGS. 6A to 6C are top views of fabrication steps for a pixel TFT, astorage capacitor and a driver circuit TFT;

FIG. 7A to 7C are top views of fabrication steps for a pixel TFT, astorage capacitor and a driver circuit TFT;

FIGS. 8A to 8C are top views of fabrication steps for a driver circuitTFT;

FIGS. 9A to 9C are top views of fabrication steps for a pixel TFT;

FIG. 10 is a top view of the I/O terminal and wiring circuit layout of aliquid crystal display device;

FIG. 11 is a cross-sectional view of the construction of a liquidcrystal display device;

FIG. 12 is a perspective view of the construction of a liquid crystaldisplay device;

FIG. 13 is a top view of pixels in a display region;

FIG. 14 is a circuit block diagram for a liquid crystal display device;

FIGS. 15A to 15 C are sets of illustrations showing positionalrelationships between gate electrodes and LDD regions;

FIGS. 16A to 16C are sets of illustrations showing connections betweengate electrodes and gate wirings;

FIGS. 17A to 17E are sets of illustrations showing examples ofsemiconductor devices;

FIGS. 18A and 18B are a top view and a cross sectional view of an ELdisplay device, respectively;

FIGS. 19A and 19B are cross sectional views of a pixel portion of an ELdisplay device;

FIGS. 20A and 20B are a top view and a circuit diagram of a pixelportion of an EL display device;

FIGS. 21A to 21C are circuit diagrams of a pixel portion of an ELdisplay device;

FIG. 22 is a cross sectional photograph of a contact portion of a gateelectrode and a gate wiring taken by transmission electron microscope;

FIG. 23 is a cross sectional photograph of an interface in a gateelectrode (Ta) and a gate wiring (Al—Nd) taken by transmission electronmicroscope;

FIGS. 24A and 24B are V_(G)-I_(D) characteristic and an investigation bybias-thermal stress test of TFT;

FIGS. 25A and 25B are differences in waveform between the signal inputsection and the terminal section, where 25A shows the rise of thewaveform and 25B shows the fall of the waveform; and

FIGS. 26A and 26B are calculative simulations of contact resistance in agate electrode and a gate wiring.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment Mode 1

Embodiment of the present invention will now be explained with referenceto FIGS. 1 to 5. The detailed description that follows will deal withthe steps of a process whereby the pixel TFT in the display region andthe driver circuit TFT formed around the display region are fabricatedon the same substrate. To simplify the explanation, however, the drivercircuit will be illustrated with a shift register circuit, a CMOScircuit as the basic circuit, such as a buffer circuit, and an n-channelTFT forming a sampling circuit.

For FIG. 1A, a low alkali glass substrate or a quartz substrate may beused as the substrate 101. In this embodiment, a low alkali glasssubstrate was used. It may be heat treated beforehand at a temperatureabout 10-20° C. lower than the glass strain temperature. On the surfaceof the substrate 101 on which the TFT is formed, there is formed a basefilm 102 such as a silicon oxide film, silicon nitride film or siliconoxynitride film, in order to prevent diffusion of the impurity from thesubstrate 101. For example, the plasma CVD method is used to form alaminate of a silicon oxynitride film made from SiH₄, NH₃ and N₂O to athickness of 100 nm and a silicon oxynitride film made from SiH₄ and N₂Oto a thickness of 200 nm.

Next, a semiconductor film 103 a having an amorphous structure with athickness of 20-150 nm (preferably 30-80 nm) is formed by a publiclyknown method such as plasma CVD or sputtering. In this embodiment, anamorphous silicon film was formed to a thickness of 55 nm by plasma CVD.Semiconductor films with amorphous structures include amorphoussemiconductor films and fine crystalline semiconductor films, and acompound semiconductor film with an amorphous structure, such as anamorphous silicon-germanium film, may also be used. Since the base film102 and the amorphous silicon film 103 a can be formed by the same filmforming method, they may be made by continuous formation. After formingthe base film, contamination of the surface can be prevented by onceremoving it from the air atmosphere, thus reducing fluctuation of theTFT properties and variation in the threshold voltage of the fabricatedTFT (FIG. 1A).

A publicly known crystallizing technique is then used to form acrystalline silicon film 103 b from the amorphous silicon film 103 a.For example, a laser crystallizing or heat crystallizing method (solidphase growth method) may be used, and here a crystalline silicon film103 b was formed by a crystallization method using a catalyst element,according to the technique disclosed in Japanese Laid-Open PatentPublication No. 7-130652. Before the crystallization step, although itwill depend on the moisture content of the amorphous silicon film, heattreatment is preferably effect for about one hour at 400-500° C. toreduce the moisture content to under 5 atom % prior to crystallization.Crystallization of the amorphous silicon film causes rearrangement ofthe atoms to a more dense form, so that the thickness of the crystallinesilicon film that is fabricated is reduced by about 1-15% from thethickness of the original amorphous silicon film (55 nm in thisembodiment) (FIG. 1B).

The crystalline silicon film 103 b is then separated into insularsections to form insular semiconductor layers 104-107. A mask layer 108is then formed by a silicon oxide film with a thickness of 50-100 nm byplasma CVD or sputtering (FIG. 1C).

A resist mask 109 was provided, and boron (B) was added as a p-typeimpurity element at a concentration of about 1×10¹⁶ to 5×10¹⁷ atoms/cm³for the purpose of limiting the threshold voltage of the insularsemiconductor layers 105-107 forming the n-channel TFT. The addition ofboron (B) may be accomplished by an ion doping method, or it may beadded simultaneously with formation of the amorphous silicon film. Whilethe addition of boron (B) is not necessarily essential, thesemiconductor layers 110-112 were preferably formed with boron (B) addedthereto to keep the threshold voltage of the n-channel TFT in theprescribed range (FIG. 1D).

To form the LDD regions of the n-channel TFT of the driver circuit, ann-type impurity element is selectively added to the insularsemiconductor layers 110, 111. A resist mask 113-116 is formedbeforehand for this purpose. The n-type impurity element used may bephosphorus (P) or arsenic (As), and in this case an ion doping methodwas employed using phosphine (PH₃) for addition of phosphorus (P). Thephosphorus (P) concentration of the formed impurity regions 117, 118 maybe in the range of 2×10¹⁶ to 5×10¹⁹ atoms/cm³. Throughout the presentspecification, the concentration of the n-type impurity element in theimpurity regions 117-119 formed here will be represented as (n⁻). Theimpurity region 119 is a semiconductor layer for formation of thestorage capacitor of the pixel matrix circuit, and phosphorus (P) wasadded at the same concentration in this region as well (FIG. 2A).

This is followed by a step of removing the mask layer 108 byhydrofluoric acid or the like, and activating the impurity elementsadded in FIG. 1D and FIG. 2A. The activation may be carried out by heattreatment for 1-4 hours at 500-600° C. in a nitrogen atmosphere, or by alaser activation method. These may also be carried out in combination.In this embodiment, a laser activation method was used, with KrF excimerlaser light (248 nm wavelength) to form a linear beam, for scanning atan oscillation frequency of 5-50 Hz and an energy density of 100-500mJ/cm² with 80-98% linear beam overlap, to treat the entire substrate onwhich the insular semiconductor layers had been formed. There are noparticular restrictions on the laser light irradiation conditions, andthey may be appropriately set by the operator.

A gate insulating film 120 is then formed with a silicon-containinginsulating film to a thickness of 10-150 nm using plasma CVD orsputtering. For example, a silicon oxynitride film is formed to athickness of 120 nm. The gate insulating film may also be a single layeror multi-layer structure of other silicon-containing insulating films(FIG. 2B).

A first conductive layer is then made to form the gate electrodes. Thisfirst conductive layer may be formed as a single layer, but if necessaryit may also have a laminated structure of two or three layers. In thisembodiment, a conductive layer (A) 121 made of a conductive metalnitride film and a conductive layer (B) 122 made of a metal film werelaminated. The conductive layer (B) 122 may be formed of an elementselected from among tantalum (Ta), titanium (Ti), molybdenum (Mo) andtungsten (W), or an alloy composed mainly of one of these elements, oran alloy film comprising a combination of these elements (typically aMo—W alloy film or Mo—Ta alloy film), and the conductive layer (A) 121is formed of tantalum nitride (TaN), tungsten nitride (WN), titaniumnitride (TiN) or molybdenum nitride (MoN). As alternative materials forthe conductive layer (A) 121 there may be used tungsten silicide,titanium silicide or molybdenum silicide. The conductive layer (B) mayhave a reduced impurity concentration for the purpose of lowerresistance, and in particular the oxygen concentration was satisfactoryat under 30 ppm. For example, tungsten (W) with an oxygen concentrationof under 30 ppm allowed realization of a resistivity of under 20 μΩcm.

The conductive layer (A) 121 may be 10-50 nm (preferably 20-30 nm) andthe conductive layer (B) 122 may be 200-400 nm (preferably 250-350 nm).In this embodiment, a tantalum nitride film with a thickness of 30 nmwas used as the conductive layer (A) 121 and a Ta film of 350 nm wasused as the conductive layer (B) 122, and both were formed bysputtering. In this film formation by sputtering, addition of anappropriate amount of Xe or Kr to the Ar sputtering gas can alleviatethe internal stress of the formed film to thus prevent peeling of thefilm. Though not shown, it is effective to form a silicon film dopedwith phosphorus (P) to a thickness of about 2-20 nm under the conductivelayer (A) 121. This can improve adhesion and prevent oxidation of theconductive film formed thereover, while also preventing diffusion oftrace alkali metal elements in the conductive layer (A) or conductivelayer (B) into the gate insulating film 120 (FIG. 2C).

A resist mask 123-127 is then formed, and the conductive layer (A) 121and conductive layer (B) 122 are etched together to form gate electrodes128-131 and a capacitor wiring 132. The gate electrodes 128-131 andcapacitor wiring 132 comprise the integrally formed sections 128 a-132 aconsisting of conductive layer (A) and sections 128 b-132 b consistingof conductive layer (B). Here, the gate electrodes 129, 130 formed inthe driver circuit are formed so as to overlap with a portion of theimpurity regions 117, 118 via the gate insulating layer 120 (FIG. 2D).

This is followed by a step of adding a p-type impurity element to formthe p-channel TFT source region and drain region of the driver circuit.Here, the gate electrode 128 is used as a mask to form self-aligningimpurity regions. The region in which the n-channel TFT is formed iscovered at this time with a resist mask 133. The impurity regions 134are formed by an ion doping method using diborane (B₂H₆). The boron (B)concentration of this region is 3×10²⁰ to 3×10²¹ atoms/cm³. Throughoutthis specification, the concentration of the p-type impurity element inthe impurity regions 134 formed here will be represented as (p⁺) (FIG.3A).

Next, impurity regions functioning as a source region or drain regionwere formed in the n-channel TFT. Resist masks 135-137 were formed, andan n-type impurity element was added to form impurity regions 138-142.This was accomplished by an ion doping method using phosphine (PH₃), andthe phosphorus (P) concentration in the regions was in the range of1×10²⁰ to 1×10²¹ atoms/cm³. Throughout the present specification, theconcentration of the n-type impurity element in the impurity regions138-142 formed here will be represented as (n⁺) (FIG. 3B).

The impurity regions 138-142 already contain phosphorus (P) or boron (B)added in the previous step, but since a sufficiently high concentrationof phosphorus (P) is added in comparison, the influence of thephosphorus (P) or boron (B) added in the previous step may be ignored.As the concentration of phosphorus (P) added to the impurity region 138is ½ to ⅓ of the boron (B) concentration added in FIG. 3A, the p-typeconductivity is guaranteed so that there is no effect on the propertiesof the TFT.

This was followed by a step of adding an n-type impurity to form an LDDregion in the n-channel TFT of the pixel matrix circuit. Here, the gateelectrodes 131 were used as a mask for self-aligning addition of ann-type impurity element by an ion doping method. The concentration ofphosphorus (P) added was 1×10¹⁶ to 5×10¹⁸ atoms/cm³, and addition of alower concentration than the concentrations of the impurity elementsadded in FIG. 2A, FIG. 3A and FIG. 3B, substantially forms only impurityregions 143, 144. Throughout this specification, the concentration ofthe n-type impurity element in these impurity regions 143, 144 will berepresented as (n⁻⁻) (FIG. 3C).

This was followed by a step of heat treatment for activation of then-type or p-type impurity element added at their respectiveconcentrations. This step can be accomplished by the furnace annealmethod, laser anneal method or rapid thermal anneal method (RTA method).Here, the activation step was accomplished by the furnace anneal method.The heat treatment is carried out in a nitrogen atmosphere with anoxygen concentration of no greater than 1 ppm and preferably no greaterthan 0.1 ppm, at 400-800° C. and typically 500-600° C., and for thisembodiment the heat treatment was carried out at 550° C. for 4 hours.When a heat resistant material such as a quartz substrate is used forthe substrate 101, the heat treatment may even be at 800° C. for onehour, and this allowed activation of the impurity element and formationof a satisfactory bond between the impurity element-added impurityregion and the channel-forming region.

In the heat treatment, conductive layers (C) 128 c-132 c are formed to athickness of 5-80 nm from the surfaces of the metal films 128 b-132 bforming the gate electrodes 128-131 and the capacitor wiring 132. Forexample, when the conductive layers (B) 128 b-132 b are of tungsten (W),tungsten nitride (WN) is formed, whereas when tantalum (Ta) is used,tantalum nitride (TaN) may be formed. The conductive layers (C) 128c-132 c may be formed in the same manner by exposing the gate electrodes128-131 to a nitrogen-containing plasma atmosphere, using eithernitrogen or ammonia. A step was also performed for hydrogenation of theinsular semiconductor layer by heat treatment at 300-450° C. for 1-12hours in an atmosphere containing 3-100% hydrogen. This step is a stepfor terminating the dangling bond of the semiconductor layer bythermally excited hydrogen. Plasma hydrogenation (using plasma-excitedhydrogen) may also be carried out as another means for hydrogenation.

When the insular semiconductor layer was fabricated by a method ofcrystallization from an amorphous silicon film using a catalyst element,the catalyst element remained in a trace amount in the insularsemiconductor layers. While the TFT can of course be completed even inthis condition, it is more preferable for the residual catalyst elementto be eliminated at least from the channel-forming region. One meansused to eliminate the catalyst element was utilizing the getteringeffect by phosphorus (P). The phosphorus (P) concentration necessary forgettering is on the same level as the impurity region (n⁺) formed inFIG. 3B, and the heat treatment for the activation step carried out hereallowed gettering of the catalyst element from the channel-formingregion of the n-channel TFT and p-channel TFT (FIG. 3D).

FIG. 6A and FIG. 7A are top views of a TFT up to this step, wherecross-section A-A′ and cross-section C-C′ correspond to A-A′ and C-C′ inFIG. 3D. Cross-section B-B′ and cross-section D-D′ correspond to thecross-sectional views of FIG. 8A and FIG. 9A. The top views of FIGS. 6Ato 6C and FIGS. 7A to 7C omit the gate electrode films, but up to thisstep, at least the gate electrodes 128-131 and capacitor wiring 132 areformed on the insular semiconductor layers 104-107, as shown.

After completion of the steps of activation and hydrogenation, thesecond conductive layer to serve as the gate wiring is formed. Thissecond conductive layer may be formed with a conductive layer (D)composed mainly of aluminum (Al) or copper (Cu) as low resistancematerials, and a conductive layer (E) made of titanium (Ti), tantalum(Ta), tungsten (W) or molybdenum (Mo). In this embodiment, theconductive layer (D) 145 was an aluminum (Al) film containing 0.1-2 wt %titanium (Ti), and the conductive layer (E) 146 was a titanium (Ti)film. The conductive layer (D) 145 may be formed to 200-400 nm(preferably 250-350 nm), and the conductive layer (E) 146 may be formedto 50-200 nm (preferably 100-150 nm) (FIG. 4A).

The conductive layer (E) 146 and conductive layer (D) 145 were subjectedto etching treatment to form the gate wiring connecting the gateelectrodes, thus forming gate wirings 147, 148 and capacitor wiring 149.The etching treatment first accomplished removal from the surface of theconductive layer (E) to partway through the conductive layer (D) by adry etching method using a mixed gas of SiCl₄, Cl₂ and BCl₃, and thenwet etching was performed with a phosphoric acid-based etching solutionto remove the conductive layer (D), thus allowing formation of a gatewiring while maintaining selective working with the base layer.

FIG. 6B and FIG. 7B are top views of this state, where cross-sectionA-A′ and cross-section C-C′ correspond to A-A′ and C-C′ in FIG. 4B.Cross-section B-B′ and cross-section D-D′ correspond to the B-B′ andD-D′ in FIG. 8B and FIG. 9B. In FIG. 6B and FIG. 7B, part of the gatewirings 147, 148 overlap and are in electrical contact with part of thegate electrodes 128, 129, 131. This condition is clearly shown in thecross-sectional structural diagrams of FIG. 8B and FIG. 9B correspondingto cross-section B-B′ and cross-section D-D′, where conductive layer (C)forming the first conductive layer and conductive layer (D) forming thesecond conductive layer are in electrical contact.

A first interlayer insulating film 150 is formed with a silicon oxidefilm or silicon oxynitride film to a thickness of 500-1500 nm, afterwhich contact holes are formed reaching to the source region or drainregion formed in each insular semiconductor layer, to form sourcewirings 151-154 and drain wirings 155-158. While not shown here, in thisembodiment the electrode has a three-layer laminated structure withcontinuous formation of a Ti film to 100 nm, a Ti-containing aluminumfilm to 300 nm and a Ti film to 150 nm by sputtering.

Next, a silicon nitride film, silicon oxide film or a silicon oxynitridefilm is formed to a thickness of 50-500 nm (typically 100-300 nm) as apassivation film 159. Hydrogenation treatment in this state gavefavorable results for enhancement of the TFT characteristics. Forexample, heat treatment may be carried out for 1-12 hours at 300-450° C.in an atmosphere containing 3-100% hydrogen, or a similar effect may beachieved by using a plasma hydrogenation method. Here, an opening may beformed in the passivation film 159 at the position where the contactholes are to be formed for connection of the pixel electrodes and thedrain wirings (FIG. 4C).

FIG. 6C and FIG. 7C show top views of this condition, wherecross-section A-A′ and cross-section C-C′ correspond to A-A′ and C-C′ inFIG. 4C. Cross-section B-B′ and cross-section D-D′ correspond to B-B′and D-D′ in FIG. 8C and FIG. 9C. FIG. 6C and FIG. 7C do not show thefirst interlayer insulating film, but the source wirings 151, 152, 154and drain wirings 155, 156, 158 in the source and drain regions (notshown) of the insular semiconductor layers 104, 105, 107 are connectedvia contact holes formed in the first interlayer insulating film.

Next, a second interlayer insulating film 160 made of an organic resinis formed to a thickness of 1.0-1.5 μm. The organic resin used may bepolyimide, acryl, polyamide, polyimideamide, BCB (benzocyclobutene) orthe like. Here, after coating onto the substrate, a thermalpolymerization type polyimide was used for formation by firing at 300°C. A contact hole reaching to the drain wiring 158 is then formed in thesecond interlayer insulating film 160, and pixel electrodes 161, 162 areformed. The pixel electrodes used may be of a transparent conductivefilm in the case of a transmitting liquid crystal display device, or ofa metal film in the case of a reflective liquid crystal display device.In this embodiment a transmitting liquid crystal display device wasused, and therefore an indium-tin oxide (ITO) film was formed bysputtering to a thickness of 100 nm (FIG. 5).

A substrate with a driver circuit TFT and a display region pixel TFT onthe same substrate was completed in this manner. A p-channel TFT 201, afirst n-channel TFT 202 and a second n-channel TFT 203 were formed onthe driver circuit and a pixel TFT 204 and a storage capacitor 205 wereformed on the display region. Throughout the present specification, thissubstrate will be referred to as an active matrix substrate forconvenience.

The p-channel TFT 201 of the driver circuit has a channel-forming region206, source regions 207 a, 207 b and drain regions 208 a, 208 b in theinsular semiconductor layer 104. The first n-channel TFT 202 has achannel-forming region 209, an LDD region 210 overlapping the gateelectrode 129 (hereunder this type of LDD region will be referred to asL_(ov)), a source region 211 and a drain region 212 in the insularsemiconductor layer 105. The length of this L_(ov) region in the channellength direction was 0.5-3.0 μm, and is preferably 1.0-1.5 μm. Thesecond n-channel TFT 203 has a channel-forming region 213, LDD regions214, 215, a source region 216 and a drain region 217 in the insularsemiconductor layer 106. These LDD regions are formed of an L_(ov),region and an LDD region not overlapping the gate electrode 130(hereunder this type of LDD region will be referred to as L_(off)), andthe length of this L_(off) region in the channel length direction is0.3-2.0 μm, and preferably 0.5-1.5 μm. The pixel TFT 204 haschannel-forming regions 218, 219, L_(off) regions 220-223 and source ordrain regions 224-226 in the insular semiconductor layer 107. The lengthof the L_(off) regions in the channel length direction is 0.5-3.0 μm,and preferably 1.5-2.5 μm. The capacitor wirings 132, 149 and aninsulating film made of the same material as the gate insulating filmare connected to the drain region 226 of the pixel TFT 204, and astorage capacitor 205 is formed from an n-type impurity element-addedsemiconductor layer 227. In FIG. 5 the pixel TFT 204 has a double gatestructure, but it may also have a single gate structure, and there is noproblem with a multi-gate structure provided with multiple gateelectrodes.

Thus, the present invention optimizes the structures of the TFTs of eachcircuit in accordance with the specifications required for the pixel TFTand driver circuit, thus allowing the operating performance andreliability of the semiconductor device to be improved. In addition, byforming the gate electrodes with a heat resistant conductive material,it is possible to facilitate activation of the LDD regions and sourceand drain regions, and thus adequately reduce wiring resistance byformation of the gate wirings with low resistance materials. This allowsapplication to display devices having display regions (screen sizes) inthe class of 4 inches or larger.

Embodiment Mode 2

FIGS. 16A to 16C show other embodiments of gate electrodes and gatewirings. The gate electrodes and gate wirings in FIG. 16 are formed inthe same manner as the steps indicated for Embodiment mode 1, and areformed over the insular semiconductor layer 901 and gate insulating film902.

In FIG. 16A, the first conductive layer as the gate electrode is aconductive layer (A) 903 formed of tantalum nitride (TaN), tungstennitride (WN), titanium nitride (TiN) or molybdenum nitride (MoN). Theconductive layer (B) 904 is formed of an element selected from amongtantalum (Ta), titanium (Ti), molybdenum (Mo) and tungsten (W), or analloy composed mainly of the element or an alloy film comprising acombination of those elements, while a conductive layer (C) 905 isformed on the surface in the same manner as Embodiment mode 1. Theconductive layer (A) 903 may be 10-50 nm (preferably 20-30 nm) and theconductive layer (B) 904 may be 200-400 nm (preferably 250-350 nm). Thesecond conductive layer as the gate wiring is formed by laminatingconductive layer (D) 906 composed mainly of aluminum (Al) or copper (Cu)as a low resistance material and conductive layer (E) 907 thereoverformed of titanium (Ti) or tantalum (Ta). Because aluminum (Al) andcopper (Cu) diffuse readily by stress migration or electromigration, thesilicon nitride film 908 must be formed to a thickness of 50-150 nm soas to cover the second conductive layer.

FIG. 16B shows a gate electrode and gate wiring fabricated in the samemanner as Embodiment mode 1, and a silicon film 909 doped withphosphorus (P) is formed under the gate electrode. The silicon film 909doped with phosphorus (P) has the effect of preventing diffusion oftrace alkali metal elements in the gate electrode into the gateinsulating film, and is useful for the purpose of guaranteeing thereliability of the TFT.

FIG. 16C is an example of formation on the first conductive layerforming the gate electrode, with a silicon film 910 doped withphosphorus (P). The silicon film doped with phosphorus (P) is a higherresistance material than the other conductive metal material, but byforming the second conductive layer composing the gate wiring withaluminum (Al) or copper (Cu), it may be applied to large-area liquidcrystal display devices. Here, the gate wiring may be made with athree-layer structure with formation of a Ti film 911 to 100 nm, aTi-containing aluminum (Al) film 912 to 300 nm and a Ti film 913 to 150nm, avoiding direct contact between the aluminum (Al) film and thephosphorus (P)-doped silicon film to provide heat resistance.

Embodiment Mode 3

FIGS. 15A to 15C are illustrations of the structure of a TFT accordingto the invention, showing the positional relationship between the gateelectrode and LDD region in a TFT having a semiconductor layerchannel-forming region, an LDD region, a gate insulating film on thesemiconductor layer and a gate electrode on the gate insulating film.

FIG. 15A shows a construction provided with a semiconductor layer havinga channel-forming region 209, LDD region 210 and drain region 212, and agate insulating film 120 and gate electrode 129 formed thereover. TheLDD region 210 is an L_(ov) provided overlapping the gate electrode 129via the gate insulating film 120. The L_(ov) has the function ofattenuating the high electric field generated near the drain whilepreventing deterioration by hot carriers, and it can be suitably used inan n-channel TFT of a driver circuit comprising a shift registercircuit, a level shifter circuit, a buffer circuit or the like.

FIG. 15B shows a construction provided with a semiconductor layer havinga channel-forming region 213, LDD regions 215 a, 215 b and a drainregion 217, and a gate insulating film 120 and gate electrode 130 formedon the semiconductor layer. The LDD region 215 a is provided overlappingthe gate electrode 130 via the gate insulating film 120. Also, the LDDregion 215 b is an L_(off) provided without overlapping the gateelectrode 130. The L_(off) has the function of reducing the off-currentvalue, and the structure provided with the L_(ov) and L_(off) canprevent deterioration by hot carriers while also reducing theoff-current value, so that it may be suitably used in an n-channel TFTof the sampling circuit of a driver circuit.

FIG. 15C shows a semiconductor layer provided with a channel-formingregion 219, an LDD region 223 and a drain region 226. The LDD region 223is an L_(off) provided without overlapping the gate electrode 131 viathe gate insulating film 120, and it can effectively reduce theoff-current value and is therefore suitable for use in a pixel TFT. Theconcentration of the n-type impurity element in the LDD region 223 ofthe pixel TFT is preferably from ½ to 1/10 less than the concentrationin the LDD regions 210, 215 of the driver circuit.

Embodiment Mode 4

In this embodiment, the steps for fabricating an active matrix-typeliquid crystal display device from an active matrix substrate will beexplained. As shown in FIG. 11, an alignment film 601 is formed on anactive matrix substrate in the state shown in FIG. 5 fabricated inEmbodiment mode 1. A polyimide resin is often used as the alignment filmfor most liquid crystal display elements. On the opposing substrate 602on the opposite side there are formed a light shielding film 603, atransparent conductive film 604 and an alignment film 605. After formingthe alignment film, it is subjected to rubbing treatment so that theliquid crystal molecules are oriented with a consistent pretilt angle.The pixel matrix circuit and the substrate opposite the active matrixsubstrate on which the CMOS circuit has been formed are attachedtogether through a sealing material or spacer (neither shown) by apublicly known cell joining step. Next, a liquid crystal material 606 isinjected between both substrates and complete sealing is accomplishedwith a sealant (not shown). The liquid crystal material used may be anypublicly known liquid crystal material. This completes the activematrix-type liquid crystal display device shown in FIG. 11.

The structure of this active matrix-type liquid crystal display devicewill now be explained with reference to the perspective view in FIG. 12and the top view in FIG. 13. The same numerals are used in FIGS. 12 and13 for correspondence with the cross-sectional structural diagrams ofFIGS. 1 to 5 and FIG. 11. The cross-sectional structure along E-E′ inFIG. 13 corresponds to the cross-sectional diagram of the pixel matrixcircuit shown in FIG. 5.

In FIG. 12, the active matrix substrate is constructed of a displayregion 306, a scanning signal driver circuit 304 and an image signaldriver circuit 305 formed on a glass substrate 101. A pixel TFT 204 isprovided in the display region, and the driver circuit provided aroundit is constructed based on a CMOS circuit. The scanning signal drivercircuit 304 and the image signal driver circuit 305 are each connectedto the pixel TFT 204 with a gate wiring 148 and source wiring 154. Also,an FPC 731 is connected to an external I/O terminal 734 and is connectedto each driver circuit with input wirings 302, 303.

FIG. 13 is a top view showing about one pixel portion of the displayregion 306. The gate wiring 148 crosses with a semiconductor layer 107under it via a gate insulating film (not shown). Also not shown on thesemiconductor layer are a source region, drain region and an L_(off)region as an n⁻ region. A connector 163 is present between the sourcewiring 154 and the source region 224, a connector 164 is present betweenthe drain wiring 158 and the drain region 226, and a connector 165 ispresent between the drain wiring 158 and the pixel electrode 161. Astorage capacitor 205 is formed in the region where the semiconductorlayer 227 extending from the drain region 226 of the pixel TFT 204overlaps with the capacitor wirings 132, 149 via a gate electrode film.

The active matrix-type liquid crystal display device of this embodimentwas explained with the structure of Embodiment mode 1, but an activematrix-type liquid crystal display device may also be fabricated usingany combination with the construction of Embodiment mode 2.

Embodiment Mode 5

FIG. 10 is an illustration showing the arrangement of the I/O terminal,display region and driver circuit of a liquid crystal display device.The display region 306 has m gate wirings and n source wirings crossingin a matrix fashion. For example, when the pixel density is VGA (VideoGraphics Array), 480 gate wirings and 640 source wirings are formed, andfor XGA (eXtended Graphics Array) 768 gate wirings and 1024 sourcewirings are formed. The screen size of the display region has a diagonallength of 340 mm in the case of a 13-inch class display, and 460 mm inthe case of an 18-inch class display. In order to realize such a liquidcrystal display device it is necessary to form the gate wirings with alow resistance material as indicated for Embodiment mode 1 andEmbodiment mode 2.

A scanning signal driver circuit 304 and an image signal driver circuit305 are provided around the display region 306. Since the lengths ofthese driver circuit gate wirings are also necessarily longer withincreasing size of the screen of the display region, they are preferablyformed of a low resistance material as indicated for Embodiment mode 1and Embodiment mode 2, in order to realize large-sized screens.

According to the invention, the input wirings 302, 303 connecting fromthe input terminal 301 to each driver circuit may be formed of the samematerial as the gate wirings, and they can contribute to the lowerwiring resistance.

Embodiment Mode 6

FIG. 14 is an illustration of the construction of the active matrixsubstrate shown in Embodiment mode 1 or Embodiment mode 2, for adirect-view display device circuit construction. The active matrixsubstrate of this embodiment has an image signal driver circuit 1001, ascanning signal driver circuit (A) 1007, a scanning signal drivercircuit (B) 1011, a precharge circuit 1012 and a display region 1006.Throughout this specification, the term “driver circuit” will includethe image signal driver circuit 1001 and the scanning signal drivercircuit (A) 1007.

The image signal driver circuit 1001 is provided with a shift registercircuit 1002, a level shifter circuit 1003, a buffer circuit 1004 and asampling circuit 1005. The scanning signal driver circuit (A) 1007 isprovided with a shift register circuit 1008, a level shifter circuit1009 and a buffer circuit 1010. The scanning signal driver circuit (B)1011 also has the same construction.

The shift register circuits 1002, 1008 have a driving voltage of 5-16 V(typically 10 V), and the n-channel TFT of the CMOS circuit forming thiscircuit suitably has the construction shown as 202 in FIG. 5. The levelshifter circuits 1003, 1009 and buffer circuits 1004, 1010 have adriving voltage as high as 14-16 V, and a CMOS circuit including then-channel TFT 202 in FIG. 5 is suitable, as for the shift registercircuit. In these circuits, formation of the gates with a multi-gatestructure is effective for raising the voltage resistance and improvingthe circuit reliability.

The sampling circuit 1005 has a driving voltage of 14-16 V, but since itis necessary to reduce the off-current value while driving is effectedwith an alternating reverse polarity, a CMOS circuit containing then-channel TFT 203 in FIG. 5 is suitable. FIG. 5 shows only an n-channelTFT, but in an actual sampling circuit it is formed in combination witha p-channel TFT. Here, the p-channel TFT is adequate with theconstruction shown by 201 in the same drawing.

The pixel TFT 204 has a driving voltage of 14-16 V, and from thestandpoint of reduced power consumption, a further reduction in theoff-current value compared to the sampling circuit is required, andtherefore the structure preferably has an LDD (L_(off)) region providedwithout overlapping of the gate electrodes in the manner of the pixelTFT 204.

The construction of this embodiment may be easily realized byfabricating the TFT according to the steps indicated for Embodimentmode 1. In this embodiment there is only shown the construction for thedisplay region and the driver circuit, but by following the steps forEmbodiment mode 1 it is possible to form a signal processing circuitsuch as a signal splitting circuit, sub-harmonic circuit, D/A converter,γ-correction circuit, operational amplifier circuit, memory circuit orcomputational processing circuit, or a logic circuit, on the samesubstrate. Thus, the present invention can realize a semiconductordevice comprising a pixel matrix circuit and its driver circuit on thesame substrate, for example, a semiconductor device equipped with asignal driver circuit and a pixel matrix circuit.

Embodiment Mode 7

An active matrix substrate and liquid crystal display device fabricatedaccording to the present invention may be used for a variety ofelectro-optical devices. The invention may also be applied to anyelectronic instrument incorporating such an electro-optical device as adisplay medium. As electronic instruments there may be mentionedpersonal computers, digital cameras, video cameras, portable informationterminals (mobile computers, cellular phones, electronic books, etc.),navigation systems, and the like. An example of one of these is shown inFIGS. 17A to 17E.

FIG. 17A is a personal computer, which is constructed with a main body2001 provided with a microprocessor or memory, an image input device2002, a display device 2003 and a keyboard 2004. According to theinvention, the display device 2003 or another signal processing circuitmay be formed.

FIG. 17B is a video camera, which is constructed with a main body 2101,a display device 2102, a voice input device 2103, an operating switch2104, a battery 2105 and an image receiving device 2106. The inventionmay be applied to the display device 2102 or to another signal drivercircuit.

FIG. 17C is a portable data terminal, which is constructed with a mainbody 2201, an image input device 2202, an image receiving device 2203,an operating switch 2204 and a display device 2205. The invention may beapplied to the display device 2205 or to another signal driver circuit.

FIG. 17D is a player used for program-recorded recording media(hereunder referred to simply as recording media), and it is constructedwith a main body 2401, a display device 2402, a speaker 2403, arecording medium 2404 and an operating switch 2405. The recording mediumused may be a DVD (Digital Versatile Disc) or compact disc (CD), andthis allows music program reproduction and image display, as well asdisplay of data for video games (or TV games) and through the internet.The invention may satisfactorily employ the display device 2402 oranother signal driver circuit.

FIG. 17E is a digital camera, which is constructed with a main body2501, a display device 2502, an eyepiece 2503, an operating switch 2504and an image receiver (not shown). The invention may be applied to theimage device 2502 or to another signal driver circuit.

Thus, the scope of the present invention is very wide and it can beapplied to electronic instruments in a variety of fields. The electronicinstruments for these embodiments can also be realized usingconstructions with any combination of Embodiment modes 1 to 6.

Embodiment Mode 8

An example of manufacturing a spontaneous light emitting type displaypanel using electro-luminescence (EL) material (hereinafter referred toas EL display device) from an active matrix substrate similar to that ofEmbodiment mode 1 is described in the present embodiment mode. FIG. 18Ashows a top view of the EL display panel. In FIG. 18A, reference numeral10 is a substrate, 11 is a pixel section, 12 is a source side drivercircuit, and 13 is a gate side driver circuit; each driver circuitreaches a FPC 17 through wirings 14 to 16, and then connected to theexternal devices.

FIG. 18B shows a cross section corresponding at line A-A′ of FIG. 18A.Here an opposite plate 80 is disposed over at least the pixel section,preferably over driver circuit and pixel section. Opposite plate 80 isstuck by a sealing material 19 to an active matrix substrate on whichTFTs and spontaneous light emitting layer using EL material are formed.Filler (not shown in the Figure) is mixed into the sealing material 19and the two substrates are stuck together to have an approximatelyuniform distance by this filler. Further the device has a structure toseal tight with sealant 81 on the outside of sealing material 19 and thetop and the peripheral of FPC 17. Sealant 81 uses a material such assilicone resin, epoxy resin, phenol resin, or butyl rubber, etc.

A space is formed in the inside when the active matrix substrate 10 andthe opposite substrate 80 are stuck together by the sealing material 19.Fillings 83 are filled in the space. This fillings 83 also has an effectof adhering the opposite plate 80. PVC (poly vinyl chloride), epoxyresin, silicone resin, PVB (poly vinyl butyral) or EVA (ethylene vinylacetate) can be used as the fillings 83. Because the spontaneous lightemitting layer is weak against moisture and easy to deteriorate, it ispreferable to form on the inside of the fillings 83 a drying agent suchas barium oxide so that moisture absorption effect can be maintained.Further the device is structured to form a passivation film 82 over thespontaneous light emitting layer from silicon nitride film or siliconnitride oxide film etc. so that corrosion by alkali elements etc.included in the fillings 83 is prevented.

A glass plate, an aluminum plate, a stainless steel plate, an FRP(fiberglass-reinforced plastic) plate, a PVF (poly vinyl fluoride) film,a Myler film (a trademark of I.E. du Pont de Nemours and Company), apolyester film, an acrylic film or an acrylic plate can be used for theopposite plate 80. Further, moisture resistance can be increased byusing a sheet which has a structure of sandwiching an aluminum foil ofseveral tens μm with PVF film or Myler Film™. In this way, EL elementsare tightly sealed and shielded from the outer atmosphere.

In FIG. 18B, driver circuit TFT 22 (note that a CMOS circuit combiningn-channel TFT and p-channel TFT is shown in the Figure) and TFT forpixel section 23 (note that TFT which controls electric current to an ELelement is shown here) are formed over a substrate 10 and base film 21.Specifically, among these TFTs the n-channel TFT are provided with theLDD region having a structure shown in the present embodiment mode toprevent reduction of ON current due to hot carrier effect andcharacteristic deterioration due to Vth shift or bias stress.

For instance, p-channel TFT 201 and n-channel TFT 202 shown in FIG. 5may be used for driver circuit TFT 22. Though it depends on the drivingvoltage, if the driver voltage is 10V or greater, the first n-channelTFT 204 of FIG. 5 or a p-channel TFT having the similar structure may beused for pixel section TFT. While the first n-channel TFT 202 isstructured to dispose an LDD that overlaps with a gate electrode on thedrain side, it is not necessarily disposed when the driving voltage issmaller than 10V because deterioration of TFT due to hot carrier effectcan be almost neglected.

In order to fabricate an EL display device from an active matrixsubstrate in the state of FIGS. 1A to 1D, an interlayer insulating film(flattening film) 26 comprising a resin material is formed over thesource wiring and the drain wiring, and a pixel electrode 27 comprisinga transparent conductive film which is electrically connected to drainof pixel section TFT 23 is formed thereon. A compound of indium oxideand tin oxide (referred to as ITO) or a compound of indium oxide andzinc oxide can be used for the transparent conductive film. Afterforming the pixel electrode 27, an insulating film 28 is formed, and anopening section is formed over a pixel electrode 27.

Next, a spontaneous light emitting layer 29 is formed. The spontaneouslight emitting layer 29 may be a laminate structure or a single layerstructure, in which publicly known EL materials (hole injection layer,hole transport layer, light emitting layer, electron transport layer orelectron injection layer) may be freely combined. A technique of publicdomain may be utilized regarding how it is structured. Further, thereare small molecular materials and polymer materials for the EL material.Evaporation method is used in case of using a small molecular material,and a simple method such as spin coating, printing or ink jet method etccan be used in case of using a polymer material.

The spontaneous light emitting layer may be formed by an evaporationmethod utilizing a shadow mask, or ink jet method or dispenser method.In either way, a colored display is possible by forming luminescentlayers capable of emitting light of different wavelength per pixel (redlight emitting layer, green light emitting layer and blue light emittinglayer). Any other form may be used, such as combining color changinglayers (CCM) with color filters, and combining white light emittinglayers with color filters. Needless to say, a single color emitting ELdisplay device is also possible.

After forming the spontaneous light emitting layer 29, a cathode 30 isformed on top. It is preferable to remove as much as possible of themoisture and oxygen existing in the interface between the cathode 30 andspontaneous light emitting layer 29. It is therefore necessary to takemeasures such as forming the spontaneous light emitting layer 29 andcathode 30 inside a vacuum by successive film deposition, or forming thespontaneous light emitting layer 29 in an inert atmosphere and thenforming the cathode 30 without exposure to the atmosphere. It ispossible to perform the above film deposition in the present embodimentby using a multi-chamber system (cluster tool system) deposition device.

Note that a laminate structure of a LiF (lithium fluoride) film and anAl (aluminum) film is used for the cathode 30 in Embodiment mode 8.Specifically, a 1 nm thick LiF (lithium fluoride) film is formed on thespontaneous light emitting layer 29 by evaporation, and a 300 nm thickaluminum film is formed on top of that. Needless to say, a MgAgelectrode, a known cathode material, may be used. The cathode 30 isconnected to the wiring 16 in the region denoted with the referencenumeral 31. The wiring 16 is a power supply line in order to supply apreset voltage to the cathode 30, and is connected to the FPC 17 throughan anisotropic conductive paste material 32. A resin layer 80 is furtherformed on FPC 17, and adhesive strength in this section is increased.

In order to electrically connect the cathode 30 and the wiring 16 in theregion denoted as reference numeral 31, it is necessary to form acontact hole in the interlayer insulating film 26 and the insulatingfilm 28. The contact hole may be formed during etching of the interlayerinsulating film 26 (when forming the pixel electrode contact hole) andduring etching of the insulating film 28 (when forming the open sectionbefore forming the spontaneous light emitting layer). Further, etchingmay proceed in one shot all the way to the interlayer insulating film 26when etching the insulating film 28. In this case the contact holes canhave a good shape provided that the interlayer insulating film 26 andthe insulating film 28 are the same resin material.

The wiring 16 is electrically connected to FPC 17 by passing through aspace between sealing material 19 and substrate 10 (provided it isclosed by sealant 81). Note that the explanation is made here in regardto wiring 16, but other wirings 14 and 15 are also electricallyconnected to FPC 17 passing through underneath the sealing material 18in the similar way.

A more detailed cross sectional structure of the pixel section are shownhere in FIGS. 19A and 19B, top view is shown in FIG. 20A and the circuitdiagram is shown in FIG. 20B. In FIG. 19A, switching TFT 2402 providedon the substrate 2401 is formed in the same structure as pixel TFT 204of FIG. 5 of Embodiment mode 1. It becomes a structure in which 2 TFTsare connected in series by adopting double gate structure, and OFFcurrent value can be reduced by forming LDD in offset region disposednot to overlap with the gate electrode. While the present embodimentuses a double gate structure, the structure may be a triple gatestructure or a multi-gate structure having greater number of gates.

Further, a current control TFT 2403 is formed by using the firstn-channel TFT 202 shown in FIG. 5. This TFT structure is a structure inwhich LDD that overlaps with gate electrode is disposed only on thedrain side, and that increases electric current driver capacity byreducing parasitic capacitance and series resistance between gate anddrain. Also from other point of view, application of such a structurehas a very important meaning. Because current control TFT is an elementfor controlling electric current amount that flow in the EL element, itis an element which has a higher risk of deterioration due to heat andof deterioration due to hot carriers, by flow of a lot of electriccurrent. Deterioration of the current control TFT can be prevented, andthe operation stability can be increased, by providing an LDD regionthat partly overlaps with a gate electrode. In this case, drain wiring35 of switching TFT 2402 is electrically connected to gate electrode 37of current control TFT through wiring 36. The wiring denoted asreference numeral 38 is a gate wiring that electrically connects gateelectrodes 39 a and 39 b of switching TFT 2402.

Further, while the present embodiment shows a single gate structure forthe electric current TFT, 2403, it may be a multi-gate structureconnecting a plurality of TFTs in series. Moreover, it may be astructure in which a plurality of TFTs are connected in paralleldividing the channel forming region in effect, and in which the heatemission is available with high efficiency. Such structure is effectiveas a counter measure for deterioration due to heat.

As shown in FIG. 20A, the wiring which becomes the gate electrode 37 ofcurrent control TFT 2403 overlaps with the drain wiring 40 of currentcontrol TFT 2403 by interposing an insulating film in the region denotedas reference numeral 2404. A capacitor is formed here in the regiondenoted as reference numeral 2404. This capacitor 2404 functions as acapacitor to hold voltage applied to the gate of current control TFT2403. The drain wiring 40 is connected to current supply line (powersource supply line) 2501 and a constant voltage is always appliedthereto.

A first passivation film 41 is formed over the switching TFT 2402 andcurrent control TFT 2403, and a planarization film 42 comprising a resininsulating film is formed thereon. It is very important to flatten thelevel difference due to the TFT by using the planarization film 42. Aspontaneous light emitting layer to be formed later is so thin that thepresence of the level difference may sometimes cause trouble in emittinglight. Therefore flattening is desirably carried out before forming apixel electrode in order to form the spontaneous light emitting layer onthe surface as flat as possible.

Denoted by 43 is a pixel electrode (cathode of the EL element) made of aconductive film with high reflectivity, which is electrically connectedto the drain of the current controlling TFT 2403. Preferable materialfor the pixel electrode 43 is a low resistance conductive film such asan aluminum alloy film, a copper alloy film and a silver alloy film, ora lamination film of those films. Needless to say, those films may beused to form a lamination structure with other conductive films. Banks44 a and 44 b made of an insulating film (preferably resin) form agroove (corresponding to pixel) therebetween to form a light emittinglayer 44 in the groove. Though only one pixel is shown here, lightemitting layers corresponding to the colors R (red), G (green) and B(blue), respectively, may be formed. As an organic EL material forforming the light emitting layer, π conjugate polymer material is used.Representative polymer materials include a polyparaphenylene vinylene(PPV)-, polyvinyl carbazole (PVK)-, and polyfluore-based materials, etc.Among PPV-based organic EL materials of various forms, usable materialis one disclosed in, for example, H. Shenk, H. Becker, O. Gelsen, E.Kluge, W. Kreuder, and H. Spreitzer, “Polymers for Light EmittingDiodes,” Euro Display, Proceedings, 1999, pp. 33-37, or in JapanesePatent Application Laid-Open No. Hei 10-92576.

Specifically, cyanopolyphenylene vinylene is used for the light emittinglayer for emitting red light, polyphenylene vinylene is used for thelight emitting layer for emitting green light, and polyphenylenevinylene or polyalkylphenylene is used for the light emitting layer foremitting blue light. Appropriate film thickness thereof is 30 to 150 nm(preferably 40 to 100 nm). However, the description above is an exampleof an organic EL material usable as the light emitting layer and thereis no need to limit the present invention thereto. The spontaneous lightemitting layer (a layer for emitting light and for moving carriers toemit light) may be formed by freely combining the light emitting layer,electric charge transport layer and an electric charge injection layer.Instead of the polymer material that is used as the light emitting layerin the example shown in this embodiment, for instance, a small molecularorganic EL material may be used. It is also possible to use an inorganicmaterial such as silicon carbide for the electric charge transport layerand the electric charge injection layer. Known materials can be used forthese organic EL materials and inorganic materials.

The spontaneous light emitting layer in this embodiment has a laminationstructure in which a hole injection layer 46 comprising PEDOT(polytiophene) or PAni (polyaniline) is layered on the light emittinglayer 45. In the case of this embodiment, light produced in the lightemitting layer 45 is emitted toward the top face (upwards beyond theTFTs), which requires an anode having light transmissivity. Thetransparent conductive film may be formed from a compound of indiumoxide and tin oxide or a compound of indium oxide and zinc oxide, andpreferred material is one that can be formed into a film at atemperature as low as possible because the transparent conductive filmis formed after forming the light emitting layer and the hole injectionlayer which have low heat resistance.

A spontaneous light emitting element 2045 is completed upon formation ofthe anode 47. The spontaneous light emitting element 2045 here refers toa capacitor consisting of the pixel electrode (cathode) 43, the lightemitting layer 45, the hole injection layer 46 and the anode 47. Asshown in FIG. 20A, the pixel electrode 43 extends almost all over thearea of the pixel, so that the entire pixel functions as the spontaneouslight emitting element. Therefore light emitting efficiency is veryhigh, resulting in bright image display.

In this embodiment, a second passivation film 48 is further formed onthe anode 47. Preferred second passivation film 48 is a silicon nitridefilm or a silicon nitride oxide film. A purpose of this secondpassivation film is to shut the spontaneous light emitting element fromthe external with the intention of preventing degradation of the organicEL material due to oxidation as well as suppressing degassing from theorganic EL material. This enhances reliability of the EL display device.

As described above, the EL display panel of this embodiment includes thepixel section comprising pixels that has the structure as shown in FIG.20A, the switching TFT with sufficiently low OFF current value, and thecurrent controlling TFT which is strong against hot carrier injection.Thus obtained is the EL display panel that has high reliability and iscapable of excellent image display.

FIG. 19B shows an example of inverting the structure of spontaneouslight emitting layer. The current control TFT 2601 is formed by the samestructure as p-channel TFT 201 of FIG. 5. Embodiment mode 1 may bereferred regarding the fabrication method. A transparent conductive filmis used as the pixel electrode (anode) 50 in this embodiment.Specifically, a conductive film made from a compound of indium oxide andzinc oxide is used. Needless to say, a conductive film made from acompound of indium oxide and tin oxide may be used too.

After forming banks 51 a and 51 b made of an insulating film are formed,a light emitting layer 52 comprising polyvinyl carbazole is formed byapplying a solution. An electron injection layer 53 comprising potassiumacetylacetonate (denoted as acacK) and a cathode 54 made of an aluminumalloy are formed thereon. In this case, the cathode 54 functions also asa passivation film. An EL element 2602 is thus formed. In thisembodiment, light produced in the light emitting layer 52 is emitted, asindicated by the arrow in the drawing, toward the substrate on whichTFTs are formed. It is preferable to form the current control TFT 2601by p-channel TFT in case of applying the structure of the presentexample.

The EL display device shown in this embodiment can be utilized as adisplay section of electronic devices of Embodiment mode 7.

Embodiment Mode 9

This embodiment shows in FIGS. 21A to 21C, examples where a pixel has adifferent structure from the one shown in the circuit diagram of FIG.20B. In this embodiment, reference numeral 2701 denotes a source wiringof a switching TFT 2702; 2703, gate wirings of the switching TFT 2702;2704, a current controlling TFT; 2705, a capacitor; 2706 and 2708,electric current supply line; and 2707, an EL element.

FIG. 21A shows an example in which the current supply line 2706 isshared by two pixels. In other words, this example is characterized inthat two pixels are formed so as to be axisymmetric with respect to thecurrent supply line 2706. In this case, the number of current supplylines can be reduced, further enhancing the definition of the pixelsection.

FIG. 21B shows an example in which the current supply line 2708 isarranged in parallel with the gate wirings 2703. Though the currentsupply line is arranged so as not to overlap with the gate wirings 2703in FIG. 21B, the two may overlap with each other through an insulatingfilm if the lines are formed in different layers. In this case, thecurrent supply line 2708 and the gate wirings 2703 can share theiroccupying area, further enhancing the definition of the pixel section.

An example shown in FIG. 21C is characterized in that the current supplyline 2708 is arranged, similar to the structure in FIG. 21B, in parallelwith the gate wirings 2703 and, further, two pixels are formed to beaxisymmetric with respect to the current supply line 2708. It is alsoeffective to arrange the current supply line 2708 so as to overlap withone of the gate wirings 2703. In this case, the number of current supplylines can be reduced, further enhancing the definition of the pixelsection. Though a capacitor 2705 is provided in order to hold voltageapplied onto the gate of current control TFT; 2704 in FIGS. 21A and 21B,it is possible to omit capacitor 2705.

Because n-channel TFT of the present invention as shown in FIG. 19A isused as the current control TFT 2704, it has an LDD region provided tooverlap with the gate electrode by interposing a gate insulating film.In general a parasitic capacitance called a gate capacitance is formedin this overlapped region, and the present embodiment is characterizedin that it uses the parasitic capacitance in place of a capacitor 2705.Because the capacitance of this parasitic capacitance varies by theoverlapped area of the gate electrode and the LDD region, it isdetermined by the length of the LDD region included in the overlappedregion. Further, it is possible to omit capacitor 2705 in the structuresof FIG. 21A to 21C, similarly.

Note that the circuit structure of an EL display device shown in thepresent embodiment mode may be selected from the structure of TFTs shownin Embodiment mode 1 to form a circuit shown in FIGS. 21A to 21C. It ispossible to use an EL display panel of the present embodiment as adisplay section of the electronic devices of Embodiment mode 7.

EMBODIMENTS Embodiment 1

As shown in Embodiment mode 1, the gate electrode and the gate wiring ofa TFT contact at the outside of an island semiconductor layer withoutinterposing a contact hole. The results of evaluating the resistance ofthe gate electrode and the gate wiring in such structure, are shown inTables 1 and 2. Table 1 shows sheet resistance of materials that formthe gate electrode and the gate wiring.

TABLE 1 various sheet resistances of metals for gate and gate bus linesheet resistance metallic material Film thickness (Å) (Ω/□) TaN\Ta 500\3500 1.58 w 4000 0.36 Al—Nd 2500 0.19 TaN\Ta\Al—Nd 500\3500\25000.16 W\Al—Nd 4000\2500 0.12

Table 2 shows the results of calculating contact resistance per contactsection from the measured value from a contact chain (number of contacts100 to 200), which was fabricated in order to evaluate the contactresistance of the gate electrode and the gate wiring. The area of eachcontact section is set at 4 μm×10 μm or 6 μm×10 μm.

TABLE 2 contact resistances per contact chain between gate metal andgate bus line TaN\Ta gate W gate mask design value electrode electrode(width × length × resistance resistance resistance resistance number ofcontact) (Ω) (Ω) (Ω) (Ω) 4 μm × 10 μm × 100 162.7 158.5 0.09 0.08 4 μm ×10 μm × 200 162.2 156.4 0.06 0.06 6 μm × 10 μm × 100 183.7 175.1 0.050.05 6 μm × 10 μm × 200 172.0 168.3 0.04 0.04

For the gate electrode, 2 kinds of films, namely, a laminate film of TaNfilm and Ta film, and a W film were fabricated. Gate wiring was formedfrom Al. Note that Nd is added 1% by weight to the Al. (Hereinafterdenoted as Al—Nd film.) When the overlapped area of the gate electrodeand gate wiring is presumed to be 40 μm², the contact resistance wasapproximately 200Ω for the laminate film of TaN film and Ta film, andapproximately 0.1Ω for the W film.

FIG. 22 shows the result of observing the overlapped section of a gateelectrode formed by laminating TaN film and Ta film, and Al—Nd film bytransmission electron microscope (TEM). FIG. 23 is an enlargement at theinterface between Ta Film and Al—Nd film, and the components weredetected by energy dispersion X-ray spectroscopy (EDX) at the pointsdenoted as *1 to *4 in the Figure. As a result, though Al is detected at*1 and Ta at *4, it was found that layers that include an oxide wereformed because Al and oxygen was detected at *2, and Ta and oxygen wasdetected at *3. The cause is presumed that the surface of Ta film isoxidized in the heat treatment process for impurity element activationthat was performed after forming Ta film as a gate electrode. When Al—Ndfilm is further formed, oxygen in the surface of Ta film presumablyoxidized the Al—Nd film. Such increase in the contact resistance was aresult noticeably appeared when Ta was used.

However, by testing by simulation the influence that was imposed on thesignal waveform by the contact resistance, it was confirmed that it didnot so much affect at contact resistance around 200Ω. FIGS. 26A and 26Bshow the difference due to the resistance in the rise of the waveformand in the fall of the waveform. The equivalent circuit used forcalculation is shown inserted in the Figures. The simulation was madehere by varying R2 which corresponds to contact resistance from 1Ω to1MΩ and it was confirmed that the influence by the contact resistancewas scarcely found up to approximately 10 kΩ.

Further, conduction test was performed as the reliability test of thecontact section, and difference in the contact resistance was examined.Test samples having contact section area 40 μm² and contact number 200were fabricated, and 1 mA electric current was conducted for 1 hour inthe atmosphere at 180° C. Though difference in contact resistance wastested for the gate electrode materials of above stated 2 kinds, thedifference was scarcely observed.

Embodiment 2

The reliability of fabricated TFT was investigated by bias-thermalstress test (hereinafter denoted as BT test). The size of the TFT waschannel length 8 μm and channel width 8 μm. The conditions for the testwas that gate voltage of +20V and gate voltage of 0V was applied ton-channel TFT and held under 150° C. for 1 hour. FIGS. 24A and 24B showthe result for n-channel TFT and p-channel TFT respectively butdegradation due to bias stress was scarcely observed in either case.

Embodiment 3

Influence of signal delay due to difference of gate wiring material wasevaluated. FIGS. 25A and 25B show the difference in the waveform betweenthe signal input section and the terminal section, where 25A shows therise of the waveform and 25B shows the fall of the waveform. Thedistance between the input section and the terminal section is 83 mm. InFIGS. 25A and 25B the characteristic denoted as J2 is for gate wiringwhich was formed from lamination of TaN film and Ta film, and a sampledenoted as J4 is a sample formed with a gate wiring of Al—Nd film. Thewidth of the gate wiring is 10 μm. While the former sample has a largedifference between the input section and the terminal section for risingtime and falling time, the latter sample has a very small difference.The delay time of J2 sample was approximately 10 times as much as thatof J4 sample, and as clear from sheet resistance shown in Table 1 it canbe presumed that the resistance of the wiring material affect the delaytime.

TABLE 3 unit: nsec. rise fall J4 J2 J4 J2 structure structure structurestructure input section 115 26 51 27 terminal section 170 506 74 292difference in delay 55 480 23 265

From the results above, it was shown that it is necessary to form gatewiring connected to the gate electrode, from a low resistance materiallike as in the present invention in case that the display size is 4inches or larger.

By using the present invention it is possible to dispose a TFT withperformance suited for the specifications required for functionalcircuits in a semiconductor device (specifically an electro-opticaldevice in this instance) having multiple functional circuits formed onthe same substrate, thus allowing vast improvement in the operatingcharacteristics and reliability. In particular, by forming the LDDregion of the n-channel TFT of the pixel matrix circuit with an n⁻⁻concentration and as an L_(off) alone, the off-current value can bedrastically decreased, to contribute to lower power consumption of thepixel matrix circuit. Moreover, by forming the LDD region of then-channel TFT of the driver circuit with an n⁻ concentration and as anL_(ov) alone, it is possible to increase the current driving capacityand prevent deterioration by hot carriers, to thus reduce deteriorationof the on-current value. It is also possible to improve the operationperformance and reliability of semiconductor devices (specificallyelectronic instruments in this instance) having such electro-opticaldevices as their display media.

Furthermore, by forming the gate electrodes of the pixel TFT and drivercircuit TFT with a highly heat resistant, conductive material, andforming the gate wirings connecting the gate electrodes with lowresistance materials such as aluminum (Al), it is possible to realizethe aforementioned satisfactory TFT characteristics and to use such TFTsto realize large-sized display devices of 4-inch class or larger.

1. A semiconductor device comprising: a semiconductor; a gate insulatingfilm formed over the semiconductor; a gate electrode comprising atantalum nitride layer and a tungsten layer over the tantalum nitridelayer, wherein the tantalum nitride layer is in direct contact with thegate insulating film; and an interlayer insulating film formed over thegate electrode, wherein an oxygen concentration in the tungsten layer isunder 30 ppm and a resistivity under 20 μΩcm.
 2. The semiconductordevice according to claim 1, wherein the semiconductor device is a thinfilm transistor.
 3. The semiconductor device according to claim 1,wherein the gate insulating film contains silicon.
 4. The semiconductordevice according to claim 1, wherein the tungsten layer is in contactwith the tantalum nitride layer.
 5. The semiconductor device accordingto claim 1, wherein the semiconductor is formed on an insulatingsurface.
 6. The semiconductor device according to claim 1, furthercomprising a third conductive layer in contact with the gate electrode,wherein the third conductive layer comprises a material selected fromthe group consisting of aluminum and copper.
 7. The semiconductor deviceaccording to claim 1, wherein the tungsten layer is 200-400 nm thick. 8.The semiconductor device according to claim 1, wherein the semiconductordevice is incorporated in one selected from the group consisting of apersonal computer, a video camera, a digital camera, an electronic bookand a portable information terminal.
 9. A semiconductor devicecomprising: a semiconductor; an insulating film formed over thesemiconductor; and a gate electrode comprising a tantalum nitride layerand a tungsten layer over the tantalum nitride layer, wherein thetantalum nitride layer is in direct contact with the insulating film,wherein an oxygen concentration in the tungsten layer is under 30 ppmand a resistivity under 20 μΩcm.
 10. The semiconductor device accordingto claim 9, wherein the semiconductor device is a thin film transistor.11. The semiconductor device according to claim 9, wherein theinsulating film contains silicon.
 12. The semiconductor device accordingto claim 9, wherein the tungsten layer is in contact with the tantalumnitride layer.
 13. The semiconductor device according to claim 9,wherein the semiconductor is formed on an insulating surface.
 14. Thesemiconductor device according to claim 9, further comprising a thirdconductive layer in contact with the gate electrode, wherein the thirdconductive layer comprises a material selected from the group consistingof aluminum and copper.
 15. The semiconductor device according to claimany one of claim 9, wherein the tungsten layer is 200-400 nm thick. 16.The semiconductor device according to claim 9, wherein the semiconductordevice is incorporated in one selected from the group consisting of apersonal computer, a video camera, a digital camera, an electronic bookand a portable information terminal.
 17. A semiconductor devicecomprising: a semiconductor; a gate insulating film formed over thesemiconductor; a gate electrode comprising a tantalum nitride layer anda tungsten layer over the tantalum nitride layer, wherein the tantalumnitride layer is in direct contact with the gate insulating film; and aninterlayer insulating film formed over the gate electrode, wherein thetantalum nitride layer is 10-50 nm thick, and wherein an oxygenconcentration in the tungsten layer is under 30 ppm and a resistivityunder 20 μΩcm.
 18. The semiconductor device according to claim 17,wherein the semiconductor device is a thin film transistor.
 19. Thesemiconductor device according to claim 17, wherein the gate insulatingfilm contains silicon.
 20. The semiconductor device according to claim17, wherein the tungsten layer is in contact with the tantalum nitridelayer.
 21. The semiconductor device according to claim 17, wherein thesemiconductor is formed on an insulating surface.
 22. The semiconductordevice according to claim 17, further comprising a third conductivelayer in contact with the gate electrode, wherein the third conductivelayer comprises a material selected from the group consisting ofaluminum and copper.
 23. The semiconductor device according to claim 17,wherein the tungsten layer is 200-400 nm thick.
 24. The semiconductordevice according to claim 17, wherein the semiconductor device isincorporated in one selected from the group consisting of a personalcomputer, a video camera, a digital camera, an electronic book and aportable information terminal.
 25. A semiconductor device comprising: asemiconductor; an insulating film formed over the semiconductor; and agate electrode comprising a tantalum nitride layer and a tungsten layerover the tantalum nitride layer, wherein the tantalum nitride layer isin direct contact with the insulating film, wherein the tantalum nitridelayer is 10-50 nm thick, and wherein an oxygen concentration in thetungsten layer is under 30 ppm and a resistivity under 20 μΩcm.
 26. Thesemiconductor device according to claim 25, wherein the insulating filmcontains silicon.
 27. The semiconductor device according to claim 25,wherein the semiconductor device is a thin film transistor.
 28. Thesemiconductor device according to claim 25, wherein the tungsten layeris in contact with the tantalum nitride layer.
 29. The semiconductordevice according to claim 25, wherein the semiconductor is formed on aninsulating surface.
 30. The semiconductor device according to claim 25,further comprising a third conductive layer in contact with the gateelectrode, wherein the third conductive layer comprises a materialselected from the group consisting of aluminum and copper.
 31. Thesemiconductor device according to claim 25, wherein the tungsten layeris 200-400 nm thick.
 32. The semiconductor device according to claim 25,wherein the semiconductor device is incorporated in one selected fromthe group consisting of a personal computer, a video camera, a digitalcamera, an electronic book and a portable information terminal.
 33. Asemiconductor device comprising: a semiconductor; a gate insulating filmformed over the semiconductor; a gate electrode comprising a tantalumnitride layer and a metal layer over the tantalum nitride layer, whereinthe tantalum nitride layer is in direct contact with the gate insulatingfilm; and an interlayer insulating film formed over the gate electrode,wherein an oxygen concentration in the metal layer is under 30 ppm. 34.The semiconductor device according to claim 33, wherein thesemiconductor device is a thin film transistor.
 35. The semiconductordevice according to claim 33, wherein the gate insulating film containssilicon.
 36. The semiconductor device according to claim 33, wherein themetal layer is in contact with the tantalum nitride layer.
 37. Thesemiconductor device according to claim 33, wherein the semiconductor isformed on an insulating surface.
 38. The semiconductor device accordingto claim 33, further comprising a third conductive layer in contact withthe gate electrode, wherein the third conductive layer comprises amaterial selected from the group consisting of aluminum and copper. 39.The semiconductor device according to claim 33, wherein the metal layeris 200-400 nm thick.
 40. The semiconductor device according to claim 33,wherein the semiconductor device is incorporated in one selected fromthe group consisting of a personal computer, a video camera, a digitalcamera, an electronic book and a portable information terminal.
 41. Asemiconductor device comprising: a semiconductor; an insulating filmformed over the semiconductor; and a gate electrode comprising atantalum nitride layer and a metal layer over the tantalum nitridelayer, wherein the tantalum nitride layer is in direct contact with theinsulating film, wherein an oxygen concentration in the metal layer isunder 30 ppm.
 42. The semiconductor device according to claim 41,wherein the semiconductor device is a thin film transistor.
 43. Thesemiconductor device according to claim 41, wherein the insulating filmcontains silicon.
 44. The semiconductor device according to claim 41,wherein the metal layer is in contact with the tantalum nitride layer.45. The semiconductor device according to claim 41, wherein thesemiconductor is formed on an insulating surface.
 46. The semiconductordevice according to claim 41, further comprising a third conductivelayer in contact with the gate electrode, wherein the third conductivelayer comprises a material selected from the group consisting ofaluminum and copper.
 47. The semiconductor device according to claim 41,wherein the metal layer is 200-400 nm thick.
 48. The semiconductordevice according to claim 41, wherein the semiconductor device isincorporated in one selected from the group consisting of a personalcomputer, a video camera, a digital camera, an electronic book and aportable information terminal.
 49. A semiconductor device comprising: asemiconductor; a gate insulating film formed over the semiconductor; agate electrode comprising a tantalum nitride layer and a metal layerover the tantalum nitride layer, wherein the tantalum nitride layer isin direct contact with the gate insulating film; and an interlayerinsulating film formed over the gate electrode, wherein the tantalumnitride layer is 10-50 nm thick, and wherein an oxygen concentration inthe metal layer is under 30 ppm.
 50. The semiconductor device accordingto claim 49, wherein the semiconductor device is a thin film transistor.51. The semiconductor device according to claim 49, wherein the gateinsulating film contains silicon.
 52. The semiconductor device accordingto claim 49, wherein the metal layer is in contact with the tantalumnitride layer.
 53. The semiconductor device according to claim 49,wherein the semiconductor is formed on an insulating surface.
 54. Thesemiconductor device according to claim 49, further comprising a thirdconductive layer in contact with the gate electrode, wherein the thirdconductive layer comprises a material selected from the group consistingof aluminum and copper.
 55. The semiconductor device according to claim49, wherein the metal layer is 200-400 nm thick.
 56. The semiconductordevice according to claim 49, wherein the semiconductor device isincorporated in one selected from the group consisting of a personalcomputer, a video camera, a digital camera, an electronic book and aportable information terminal.
 57. A semiconductor device comprising: asemiconductor; an insulating film formed over the semiconductor; and agate electrode comprising a tantalum nitride layer and a metal layerover the tantalum nitride layer, wherein the tantalum nitride layer isin direct contact with the insulating film, wherein the tantalum nitridelayer is 10-50 nm thick, and wherein an oxygen concentration in themetal layer is under 30 ppm.
 58. The semiconductor device according toclaim 57, wherein the semiconductor device is a thin film transistor.59. The semiconductor device according to claim 57, wherein theinsulating film contains silicon.
 60. The semiconductor device accordingto claim 57, wherein the metal layer is in contact with the tantalumnitride layer.
 61. The semiconductor device according to claim 57,wherein the semiconductor is formed on an insulating surface.
 62. Thesemiconductor device according to claim 57, further comprising a thirdconductive layer in contact with the gate electrode, wherein the thirdconductive layer comprises a material selected from the group consistingof aluminum and copper.
 63. The semiconductor device according to claim57, wherein the metal layer is 200-400 nm thick.
 64. The semiconductordevice according to claim 57, wherein the semiconductor device isincorporated in one selected from the group consisting of a personalcomputer, a video camera, a digital camera, an electronic book and aportable information terminal.
 65. A semiconductor device comprising: asemiconductor comprising a channel-forming region and a lightly-dopedregion adjacent to the channel-forming region; a gate insulating filmcomprising silicon oxynitride formed over the semiconductor; a gateelectrode comprising a metal nitride layer and a metal layer over themetal nitride layer, wherein the gate electrode overlaps thelightly-doped region; and a conductive layer in contact with the gateelectrode, wherein the conductive layer contains at least one selectedfrom the group consisting of aluminum and copper.
 66. The semiconductordevice according to claim 65, wherein the semiconductor device is a thinfilm transistor.
 67. The semiconductor device according to claim 65,wherein the metal layer is in contact with the metal nitride layer. 68.The semiconductor device according to claim 65, wherein thesemiconductor is formed on an insulating surface.
 69. The semiconductordevice according to claim 65, wherein the metal layer is 200-400 nmthick.
 70. The semiconductor device according to claim 65, wherein thesemiconductor device is incorporated in one selected from the groupconsisting of a personal computer, a video camera, a digital camera, anelectronic book and a portable information terminal.
 71. A semiconductordevice comprising: a semiconductor comprising a channel-forming regionand a lightly-doped region adjacent to the channel-forming region; agate insulating film comprising silicon oxynitride formed over thesemiconductor; a gate electrode comprising a metal nitride layer and ametal layer over the metal nitride layer, wherein the gate electrodeoverlaps the lightly-doped region; an interlayer insulating filmcomprising silicon nitride in contact with a top surface and a sidesurface of the gate electrode; and a conductive layer in contact withthe gate electrode, wherein the conductive layer contains at least oneselected from the group consisting of aluminum and copper.
 72. Thesemiconductor device according to claim 71, wherein the semiconductordevice is a thin film transistor.
 73. The semiconductor device accordingto claim 71, wherein the metal layer is in contact with the metalnitride layer.
 74. The semiconductor device according to claim 71,wherein the semiconductor is formed on an insulating surface.
 75. Thesemiconductor device according to claim 71, wherein the metal layer is200-400 nm thick.
 76. The semiconductor device according to claim 71,wherein the semiconductor device is incorporated in one selected fromthe group consisting of a personal computer, a video camera, a digitalcamera, an electronic book and a portable information terminal.
 77. Asemiconductor device comprising: a semiconductor comprising achannel-forming region and a lightly-doped region adjacent to thechannel-forming region; a gate insulating film formed over thesemiconductor; a gate electrode comprising a tantalum nitride layer anda metal layer over the tantalum nitride layer, wherein the gateelectrode overlaps the lightly-doped region; and a conductive layer incontact with the gate electrode, wherein the conductive layer containsat least one selected from the group consisting of aluminum and copper.78. The semiconductor device according to claim 77, wherein thesemiconductor device is a thin film transistor.
 79. The semiconductordevice according to claim 77, wherein the metal layer is in contact withthe tantalum nitride layer.
 80. The semiconductor device according toclaim 77, wherein the semiconductor is formed on an insulating surface.81. The semiconductor device according to claim 77, wherein the metallayer is 200-400 nm thick.
 82. The semiconductor device according toclaim 77, wherein the semiconductor device is incorporated in oneselected from the group consisting of a personal computer, a videocamera, a digital camera, an electronic book and a portable informationterminal.
 83. A semiconductor device comprising: a semiconductorcomprising a channel-forming region and a lightly-doped region adjacentto the channel-forming region; a gate insulating film formed over thesemiconductor; a gate electrode comprising a tantalum nitride layer anda metal layer over the tantalum nitride layer, wherein the gateelectrode overlaps the lightly-doped region; an interlayer insulatingfilm comprising silicon nitride in contact with a top surface and a sidesurface of the gate electrode; and a conductive layer in contact withthe gate electrode, wherein the conductive layer contains at least oneselected from the group consisting of aluminum and copper.
 84. Thesemiconductor device according to claim 83, wherein the semiconductordevice is a thin film transistor.
 85. The semiconductor device accordingto claim 83, wherein the metal layer is in contact with the tantalumnitride layer.
 86. The semiconductor device according to claim 83,wherein the semiconductor is formed on an insulating surface.
 87. Thesemiconductor device according to claim 83, wherein the metal layer is200-400 nm thick.
 88. The semiconductor device according to claim 83,wherein the semiconductor device is incorporated in one selected fromthe group consisting of a personal computer, a video camera, a digitalcamera, an electronic book and a portable information terminal.